The weapon is a "binary" weapon, which means it combines two non-poisonous chemicals to make Sarin. These chemicals are seperated by a partition in the shell. WHen the shell is fired from the gun, the partition ruptures, letting the chemicals mix during the flight to the target. WHen the shell detonates, it releases a cloud of sarin.
This shell was detonated before the chemicals mixed, resulting in most of the chemicals being dispersed before they could mix. The exposure probably happened only when both chemicals came to rest in the same spot, in isolated areas over the dispersal area. Lucky for us.
I am glad no one got hurt on this, but unfortunately it is going to take several (or hundreds) to die from one of these WMDs before the media takes notice. It's a shame.
The "precursor chemicals" are NOT real safe. One of them may be simply something such as alcohol, but in general, one is pretty nasty stuff even before being mixed with the other, and that one requires some special care. (Think agricultural pesticides)